FIG. 6 is an enlarged plan view of a surface of a substrate 102 of a liquid ejection head described in PTL 1. Although the surface of the substrate 102 of the liquid ejection head is covered by an orifice plate in which ejection orifices 107a and 107b are formed, in order to show positions of components of the substrate 102, the substrate 102 is shown passing through the orifice plate.
Rows of the ejection orifices 107a and 107b formed in the orifice plate are aligned in parallel with each other. The ejection orifices 107a and 107b are through-openings penetrating the orifice plate in the thickness direction of the substrate 102. In the substrate 102, three rows of supply ports 124a, 124ab, and 124b are formed so that each of the two rows of the ejection orifices 107a and 107b is sandwiched by two of the three rows of the supply ports 124a, 124ab, and 124b. The supply ports 124a, 124ab, and 124b penetrate the substrate plate in the thickness direction of the substrate 102 and are formed into substantially the same shape. Therefore, values of the flow resistance of the liquid in the supply ports 124a, 124ab, and 124b are substantially the same as each other.
Each of the two rows of the ejection orifices 107a and 107b are arranged at substantially the center between the rows of the supply ports adjacent to both sides of each row of the ejection orifices 107a and 107b. Values of the flow resistance of the liquid in flow passages from each supply port to each ejection orifice are also substantially the same as each other. Therefore, flows of the liquid flowing between the ejection orifices 107a and 107b and the supply ports 124a, 124ab, and 124b arranged to sandwich the ejection orifices 107a and 107b are substantially the same as each other.
Heaters 109a and 109b are provided at positions facing the ejection orifices 107a and 107b in the substrate 102. When the heaters 109a and 109b are driven, bubbles are generated in the liquid, so that the liquid is ejected from the ejection orifices.
Here, in the substrate 102, first areas where the row of the supply ports are provided are defined as areas alpha and second areas where the row of the heaters are provided are defined as areas beta. In this case, as shown in FIG. 6, the areas alpha and the areas beta are alternately arranged on the substrate 102.
In this liquid ejection head, the liquid supplied from the supply ports 124a and 124ab is supplied to near the ejection orifices 107a. The liquid supplied from the supply ports 124ab and 124b is supplied to near the ejection orifices 107b. The liquid supplied to near the ejection orifices 107a and 107b are ejected from the ejection orifices 107a and 107b to a recording medium by thermal energy generated by driving the heaters 109a and 109b. 
It is necessary to provide wiring to drive the heaters 109a and 109b in the liquid ejection head shown in FIG. 6. The heaters 109a and 109b are provided on a surface (hereinafter referred to as the surface) of the substrate 102 facing the orifice plate, so that the wiring needs to be also provided on the surface of the substrate 102. Such a configuration makes the structure of the surface of the substrate 102 complex. In other words, a wiring arrangement area for the wiring needs to be secured, so that it results in higher cost due to increasing the size of the substrate.
In order to reduce the wiring arrangement area on the surface of the substrate 102, a part of the wiring to drive the heaters 109a and 109b can be multi-layered. In order to do so, it is necessary to form through holes for conducting between the multi-layered wirings in the substrate 102. PTL 1 discloses a liquid ejection head in which through holes are provided.
FIG. 7 is an enlarged plan view of the surface of the substrate 102 of the liquid ejection head, in which through holes are formed, as described in PTL 1.
In the liquid ejection head shown in FIG. 7, the areas alpha and the areas beta are alternately arranged in the substrate 102 in the same manner as in the liquid ejection head shown in FIG. 6, However, a plurality of through holes are provided in one of the areas alpha (the area alpha in the center of FIG. 7) in the substrate 102 of the liquid ejection head shown in FIG. 7. Specifically, four through holes 132 are provided between each supply port in the row of the supply ports 124ab. 
In the liquid ejection head shown in FIG. 7, the through holes 132 are provided between each supply port 124ab, so that the supply port 124ab has a flattened opening shape smaller than that of the liquid ejection head shown in FIG. 6.
Therefore, the flow resistance of the liquid in the supply port 124ab is greater than that in the supply ports 124a and 124b. Therefore, the speed of refilling the supply ports 124ab with the liquid after the liquid is ejected (the refilling speed) is slow because the flow resistance of the liquid in the supply port 124ab increases.
When the driving frequency of the heaters 109a and 109b (corresponding to the ejection frequency of the ejection orifices) is increased, the refilling of the supply ports 124ab is not sufficiently performed. As a result, the liquid may not be sufficiently supplied to the ejection orifices 107a and 107b. 
Even when the liquid is sufficiently supplied, the flow resistance of the liquid in the supply port 124ab is greater than that in the supply ports 124a and 124b, so that bubbles generated when the heaters are driven spread to the supply ports 124a and 124b rather than to the supply port 124ab. Therefore, the ejection is performed by biased bubbles. Based on this, the direction of the liquid ejected from the ejection orifices 107a and 107b may be unstable.